Google is investigating whether one or more of its employees in China helped launch the cyber attack against it last month, according to reports.

It is thought the line of inquiry is a routine part of its investigation into the attack, which Google says was sophisticated, originated in China and targeted intellectual property and the email accounts of human rights activists.

According to Reuters news agency, citing two unidentified sources, the attack, which targeted people with access to specific parts of Google networks, might have been helped by employees in the company's offices in China. It has several hundred staff on the mainland.

"We're not commenting on rumour and speculation. This is an ongoing investigation and we simply cannot comment on the details," a Google spokeswoman said.

Security analysts told Reuters the malicious software or malware used in the attack was a modification of a trojan called Hydraq. A trojan is a hidden program allowing unauthorised access to a computer. The analysts said the sophistication in the attack was in knowing whom to attack, not the malware itself.

Chinese media have said that some Google China employees were denied access to internal networks following the statement, with others put on leave or transferred to different offices. Google said it would not comment on its business operations.

The Foreign Correspondents' Club of China said in a statement today that Gmail accounts used by journalists in at least two bureaux in Beijing had been hijacked and their emails forwarded to unknown email addresses. Last week several well-known Chinese advocates of human rights complained of the same problem.

A spokeswoman for Google said at the weekend that it was "business as usual" after rumours that the US firm had decided to shut down Google.cn.

In an interview with the Wall Street Journal last week, a Yahoo spokeswoman said the firm stood "aligned" against the "deeply disturbing" attacks and violation of user privacy.

"Alibaba Group has communicated to Yahoo that Yahoo's statement that it is 'aligned' with the position Google took last week was reckless given the lack of facts in evidence," said John Spelich, an Alibaba spokesman. "Alibaba doesn't share this view."

Yahoo owns about 40% of the Alibaba Group, which runs China's biggest online retailer, Taobao, and its largest e-commerce site, alibaba.com. Yahoo sold its stake in the Alibaba site late last year, but its stake in the group as a whole, acquired when it closed its own offices in China some years ago, remains a valuable asset.

A Yahoo spokeswoman said: "Yahoo condemns all cyber attacks regardless of origin or purpose. We are committed to protecting user security and privacy and we take appropriate action in the event of any kind of breach."

Google announced last Tuesday that it was no longer willing to censor search results on its Chinese service, citing the cyber attack as well as growing controls on the internet. But spokesmen say it has not yet stopped doing so and will continue talking to the Chinese government about whether it is possible to operate an uncensored service.

Chinese authorities have tried to play down the disagreement, with the ministry of commerce saying there are many ways to resolve the dispute. But officials have stressed that all foreign companies must abide by Chinese laws. It is hard to see how the two sides could reach agreement.

Figures released this week showed the number of China's internet users leapt almost 30% in 2009, to 384 million.

Analysis of Google's announcement that it has been hacked to gather information on human rights activists. Also, Henri Seydoux of Parrot - the company responsible for the AR Drone, a CES hit - discusses how he moved from journalism to inventing